Distributed power generation is the key to global energy sustainability. Extracting power from renewable energy sources with a distributed generation platform seems to be the only sustainable solution for future power generation. Grid-connected inverters are the interface between the renewable energy power conditioning systems and the utility grid. The grid-connected inverter is responsible for delivering high quality power to the utility grid. In particular, the control system of the inverter is responsible for injecting a high quality current into the utility grid. Regulatory standards for interconnecting renewable energy sources with utility grids impose very strict requirements on the quality of the output current. Particularly, the quality of the current relates to its harmonic contents and its angle with respect to the grid voltage. This angle is called Power Factor (PF).
The control system for the grid connected inverter is responsible for shaping the output current of the inverter to a nearly sinusoidal waveform. The reference signal for the inverter output current is a sinusoidal waveform with a proper angle with respect to the grid voltage. Ideally, the control system controls the inverter output such that the inverter output current tracks this sinusoidal waveform. The sinusoidal waveform has the same frequency as the grid frequency (i.e. the line frequency). In order to track the sinusoidal reference signal, the control loop should have a very high gain at the frequency of the reference signal (i.e. line frequency).
Proportional-Resonant (PR) controllers can be used to provide high gain at the line frequency. These PR controllers are commonly used to provide a very high gain at the frequency of the reference signal by tuning the frequency of the PR-controller to the grid frequency. Also, if a third-order LCL-filer is used at the output of the inverter, a PR-controller, along with a linear state-feedback, can be used to control the output current and to thereby damp the resonance created by the LCL-filter. FIG. 1 shows a typical closed-loop control system for a grid-connected inverter used in distributed power generation according to the prior art. In FIG. 1, a PR-controller is used to provide a high gain at the line frequency. Also in FIG. 1, a harmonic compensator is used to provide high gains at the harmonic frequencies for harmonic rejection of the inverter output current. In addition, a linear state-feedback is used to actively damp the resonance created by the LCL-filter at the output of the inverter.
The digital implementation of the PR-controller has some challenges. One of the main challenges is the effect of the approximation used to discretize the PR controller. The transfer function of the PR controller should be converted from the Laplace domain (s-domain) to the discrete domain (z-domain) for the digital implementation. This approximation creates a deviation of the resonant frequency of the PR controller and, in turn, creates a phase-shift between the output current of the inverter and the reference signal. The other problem is the accuracy required to implement the coefficient of the PR-controller in the discrete domain in order to maintain the resonant frequency at the line frequency. Due to the digital truncation, there might be a large deviation in the resonant frequency of the discrete PR-controller and the line frequency, leading to poor tracking of the reference signal by the control system.
There is therefore a need for systems, methods, or devices which address or mitigate the above issues with the prior art.